Power converter

ABSTRACT

The electric power converter includes a series converter connected in series between an AC power supply  1  and a load  6,  and having capacitors  30, 31  as a power supply for the converter, switching devices  12, 13,  and diodes  16, 17;  and a parallel converter connected in parallel to the AC power supply  1  and having the capacitors  30, 31,  switching devices  10, 11,  and diodes  14, 15.  The series converter compensate the change in the voltage of the AC power supply  1  to feed a certain voltage to the load  6,  and the parallel converter conducts charging and discharging operations to compensate the changes in the voltages of the capacitors  30  and  31  caused by the compensating operations of the series converter. The converter suppresses the change of the AC power supply voltage and feeds a certain voltage to the load at a high efficiency and low running costs.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

[0001] The present invention relates to an electric power converterhaving a specific feature in a main circuit configuration thereof forsupplying a constant voltage from an AC power supply to a load.

[0002]FIG. 10 is a block circuit diagram of a conventional electricpower converter for converting AC electric power to DC electric powerand for converting the converted DC electric power back to AC electricpower.

[0003] Referring to FIG. 10, a series circuit consisting ofsemiconductor switching devices 10 and 11 is connected to an end of anAC power supply 1 via a reactor 40. Diodes 14 and 15 are connected tothe semiconductor switching devices 10 and 11 in opposite parallel,respectively.

[0004] The semiconductor switching devices 10 and 11, controlled by PWM(pulse width modulation), work together with the diodes 14 and 15 as arectifier circuit for conducting control and conversion operations tomake voltages at capacitors 30 and 31 DC voltages while storing energyin the capacitors 30 and 31 connected in series.

[0005] A series circuit consisting of switching devices 12 and 13 isconnected to the series circuit consisting of the capacitors 30 and 31.Diodes 16 and 17 are connected to the switching devices 12 and 13 inopposite parallel, respectively. The switching devices 12 and 13 areoperated as an inverter through the PWM control to generate an arbitrarystable AC voltage from the smoothed DC voltage, and the generated ACvoltage is supplied to a load 6.

[0006] A capacitor 32 connected to both ends of the AC power supply 1works as a filter capacitor. A reactor 41 and a capacitor 33 connectedto an input side of the load 6 constitute an LC filter.

[0007] An electric power converter substantially identical to theconventional electric power converter shown in FIG. 10 has beendescribed in the following Patent Reference 1.

[0008] [Patent Reference 1]

[0009] Japanese Patent No. 3203464 (FIG. 1, Paragraphs [0003] and[0004])

[0010] The conventional electric power converter shown in FIG. 10 has aso-called double converter circuit configuration in which AC electricpower is converted to DC electric power once and the DC electric poweris converted back to AC electric power.

[0011]FIG. 11 is a diagram for explaining the principle of the circuitshown in FIG. 10. In the circuit shown in FIG. 10, the converter at aside of the AC power supply 1, having the switches devices 10 and 11 andthe diodes 14 and 15, works as a rectifier circuit. Therefore, therectifier circuit can be regarded as a parallel current source 5 throughwhich all the energy necessary for the load 6 flows, as shown in FIG.11.

[0012] Also in FIG. 10, the converter at a side of the load 6, includingthe switches devices 12 and 13 and the diodes 16 and 17, works as aninverter to supply a certain voltage to the load 6. Therefore, theinverter can be regarded as a parallel voltage source 3 through whichall the energy necessary for the load 6 flows, as shown in FIG. 11.

[0013] As described above, in the conventional electric power convertershown in FIG. 10 and the double converter type power converter disclosedin Patent Reference 1, all the energy supplied to the load flows throughboth the converter at the side of the AC power supply 1 and theconverter at the side of the load 6. Therefore, loses generated in sucha converter becomes high. As a result, the conversion efficiencydecreases and running cost increases.

[0014] Accordingly, in the present invention, the AC power supply 1 ofthe double converter is connected to the load 6 in a different way toprovide a so-called serial parallel converter configuration, in whichthe converter at the side of the load 6 works as a series converter.When the voltage of the AC power supply 1 is changed, the seriesconverter compensates only the change in the voltage, and the parallelconverter at the side of the AC power supply 1 compensates only theenergy necessary for the compensation.

[0015] In other words, an object of the present invention is to providean electric power converter with high conversion efficiency, therebyreducing running cost.

[0016] Another object of the present invention is to provide an electricpower converter capable of supplying a constant voltage to the loadwhile suppressing the voltage change of the AC power supply 1.

SUMMARY OF THE INVENTION

[0017] In order to achieve the objects described above, according to afirst aspect of the present invention, an electric power converterincludes a series converter connected in series between an AC powersupply and a load and including a capacitor as a converter power supply,and a parallel converter connected to the AC power supply in parallel.As structural features, the series converter compensates a change in avoltage of the AC power supply to maintain a voltage supplied to theload at a constant level. At the same time, the parallel converterconducts charging and discharging operations between the AC power supplyand the capacitor to compensate a change in a voltage of the capacitorcaused by the compensating operation of the series converter.

[0018] More specific aspects of the first aspect of the presentinvention will be described below in detail.

[0019] According to a second aspect of the invention, the electric powerconverter of the first aspect of the invention includes:

[0020] a first series switch circuit including a first semiconductorswitching device and a second semiconductor switching device connectedto each other in series and connected to diodes in opposite parallel,respectively;

[0021] a second series switch circuit including a third semiconductorswitching device and a fourth semiconductor switching device connectedto each other in series and connected to diodes in opposite parallel,respectively;

[0022] a series capacitor circuit including a first capacitor and asecond capacitor connected to each other in series;

[0023] a third capacitor connected to the AC power supply in parallel;

[0024] a fourth capacitor connected to the load in parallel;

[0025] a first reactor connected between one end of the AC power supplyconnected to one end of the load and a series connection point in thefirst series switch circuit; and

[0026] a second reactor connected between the other end of the load anda series connection point in the second series switch circuit;

[0027] wherein the first series switch circuit, the second series switchcircuit, and the series capacitor circuit are connected to each other inparallel to constitute a first parallel connection circuit;

[0028] the other end of the AC power supply is connected to the seriesconnection point in the series capacitor circuit;

[0029] the series capacitor circuit and the second series switch circuitconstitute the series converter; and

[0030] the series capacitor circuit and the first series switch circuitconstitute the parallel converter.

[0031] According to a third aspect of the invention, the electric powerconverter of the second aspect of the invention includes:

[0032] voltage detecting means for detecting input and output voltagesof the electric power converter and a voltage in the first parallelconnection circuit;

[0033] current detecting means for detecting a current flowing throughthe first reactor; and

[0034] controlling means for controlling the output voltage of theelectric power converter and the current flowing through the firstreactor using detected values of these detecting means.

[0035] According to a fourth aspect of the invention, the electric powerconverter of the second or third aspect of the invention includes:

[0036] a changeover switch including a first changeover contact, asecond changeover contact, and a common terminal;

[0037] wherein the other end of the load is disconnected from one end ofthe second reactor;

[0038] the first changeover contact is connected to the seriesconnection point in the series capacitor circuit;

[0039] the other end of the load is connected to the one end of thesecond reactor via the common terminal and the second changeover contactwhen the electric power converter is working normally; and

[0040] the common terminal is connected to the first changeover contactto supply a voltage from the AC power supply to the load when theelectric power converter is working abnormally.

[0041] According to a fifth aspect of the invention, the electric powerconverter of the second or third aspect of the invention includes:

[0042] an energy storage element, and charging and discharging meansconnected to the energy storage element, wherein a voltage is suppliedto the load using energy stored in the energy storage element when thepower supply voltage is working abnormally.

[0043] According to a sixth aspect of the invention, the electric powerconverter of the fourth aspect of the invention includes:

[0044] an energy storage element, and charging and discharging meansconnected to the energy storage element, wherein a voltage is suppliedto the load using energy stored in the energy storage element when thepower supply voltage is working abnormally.

[0045] According to a seventh aspect of the invention, the electricpower converter of the second or third aspect of the invention includes:

[0046] a third reactor with a tap in substitution for the secondreactor;

[0047] wherein the series connection point in the series capacitorcircuit is connected to one end of the third reactor, the seriesconnection point in the second series switch circuit is connected to theother end of the load through the other end of the third reactor, and

[0048] the other end of the AC power supply is disconnected from theseries connection point in the series capacitor circuit and is connectedto the tap terminal of the third reactor.

[0049] According to a eighth aspect of the invention, the electric powerconverter of the fourth aspect of the invention includes:

[0050] a third reactor with a tap in substitution for the secondreactor;

[0051] wherein the series connection point in the series capacitorcircuit is connected to one end of the third reactor, the seriesconnection point in the second series switch circuit is connected to theother end of the load through the other end of the third reactor, and

[0052] the other end of the AC power supply is disconnected from theseries connection point in the series capacitor circuit and is connectedto the tap terminal of the third reactor.

[0053] According to a ninth aspect of the invention, the electric powerconverter of the fifth aspect of the invention includes:

[0054] a third reactor with a tap in substitution for the secondreactor;

[0055] wherein the series connection point in the series capacitorcircuit is connected to one end of the third reactor, the seriesconnection point in the second series switch circuit is connected to theother end of the load through the other end of the third reactor, and

[0056] the other end of the AC power supply is disconnected from theseries connection point in the series capacitor circuit and is connectedto the tap terminal of the third reactor.

[0057] According to a tenth aspect of the invention, the connectionposition of the fourth capacitor in the second aspect is changed.

[0058] That is, according to the tenth aspect of the invention, theelectric power converter of the first aspect includes:

[0059] a first series switch circuit including a first semiconductorswitching device and a second semiconductor switching device connectedto each other in series and connected to diodes in opposite parallel,respectively;

[0060] a second series switch circuit including a third semiconductorswitching device and a fourth semiconductor switching device connectedto each other in series and connected to diodes in opposite parallel,respectively;

[0061] a series capacitor circuit including a first capacitor and asecond capacitor connected to each other in series;

[0062] a third capacitor connected to the AC power supply in parallel;

[0063] a first reactor connected between one end of the AC power supplyconnected to one end of the load and a series connection point in thefirst series switch circuit;

[0064] a second reactor connected between the other end of the load anda series connection point in the second series switch circuit; and

[0065] a fourth capacitor connected between the second end of the loadand the second end of the AC power supply;

[0066] wherein the first series switch circuit, the second series switchcircuit, and the series capacitor circuit are connected to each other inparallel to constitute a first parallel connection circuit;

[0067] the other end of the AC power supply is connected to the seriesconnection point in the series capacitor circuit;

[0068] the series capacitor circuit and the second series switch circuitconstitute the series converter; and

[0069] the series capacitor circuit and the first series switch circuitconstitute the parallel converter.

[0070] According to an eleventh aspect of the invention, the electricpower converter of the tenth aspect of the invention includes:

[0071] voltage detecting means for detecting input and output voltagesof the electric power converter and a voltage in the first parallelconnection circuit;

[0072] current detecting means for detecting a current flowing throughthe first reactor; and

[0073] controlling means for controlling the output voltage of theelectric power converter and the current flowing through the firstreactor using detected values of the voltage detecting means and thecurrent detecting means.

[0074] According to a twelfth aspect of the invention, the electricpower converter of the tenth or eleventh aspect of the inventionincludes:

[0075] an energy storage element, and charging and discharging meansconnected to the energy storage element, wherein a voltage is suppliedto the load using energy stored in the energy storage element when thepower supply voltage is working abnormally.

[0076] According to a thirteenth aspect of the invention, the electricpower converter of the tenth or eleventh aspect of the inventionincludes:

[0077] a third reactor with a tap in substitution for the secondreactor;

[0078] wherein the series connection point in the series capacitorcircuit is connected to one end of the third reactor, the seriesconnection point in the second series switch circuit is connected to theother end of the load through the other end of the third reactor, and

[0079] the other end of the AC power supply is disconnected from theseries connection point in the series capacitor circuit and is connectedto the tap terminal of the third reactor.

[0080] According to a fourteenth aspect of the invention, the electricpower converter of the twelfth aspect of the invention includes:

[0081] a third reactor with a tap in substitution for the secondreactor;

[0082] wherein the series connection point in the series capacitorcircuit is connected to one end of the third reactor, the seriesconnection point in the second series switch circuit is connected to theother end of the load through the other end of the third reactor, and

[0083] the other end of the AC power supply is disconnected from theseries connection point in the series capacitor circuit and is connectedto the tap terminal of the third reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0084]FIG. 1 is a block circuit diagram showing a first embodiment ofthe invention;

[0085]FIG. 2 is a diagram for explaining the principle of the embodimentshown in FIG. 1;

[0086]FIG. 3 is a wave chart for explaining an operation of a seriesconverter shown in FIG. 1;

[0087]FIG. 4 is a block circuit diagram showing a second embodiment ofthe invention;

[0088]FIG. 5 is a block circuit diagram showing a third embodiment ofthe invention;

[0089]FIG. 6 is a block circuit diagram showing a fourth embodiment ofthe invention;

[0090]FIG. 7 is a block circuit diagram showing a fifth embodiment ofthe invention;

[0091]FIG. 8 is a block circuit diagram showing a sixth embodiment ofthe invention;.

[0092]FIG. 9 is a block circuit diagram showing a seventh embodiment ofthe invention;

[0093]FIG. 10 is a block circuit diagram showing a conventionaltechnology; and

[0094]FIG. 11 is a diagram for explaining the principle of theconventional technology.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0095] Hereunder, embodiments of the present invention will be explainedin detail with reference to the accompanying drawings.

[0096]FIG. 1 is a block circuit diagram showing a first embodiment ofthe invention corresponding to first and second aspects of theinvention.

[0097] Referring to FIG. 1, a series circuit of first and secondsemiconductor switching devices 10 and 11 such as an IGBT (insulatedgate bipolar transistor) connected to first and second diodes 14 and 15in opposite parallel (referred to as a first series switch circuit), aseries circuit of third and fourth semiconductor switching devices 12and 13 connected to third and fourth diodes 16 and 17 in oppositeparallel (referred to as a second series switch circuit), and a seriescircuit of first and second capacitors 30 and 31 (referred to as aseries capacitor circuit) are connected to each other in parallel. Athird capacitor 32 is connected to an AC power supply 1 in parallel, anda fourth capacitor 33 is connected to a load 6 in parallel.

[0098] One end of the AC power supply 1 is connected to one end of theload 6, and the other end of the AC power supply 1 is connected to aseries connection point of the capacitors 30 and 31. A connection pointof the AC power supply 1 and the load 6 is connected to a seriesconnection point of the first and second switching devices 10 and 11 viaa first reactor 40. The other end of the load 6 is connected to a seriesconnection point of the third and fourth switching devices 12 and 13 viaa second reactor 41.

[0099] In the circuit configuration described above, when the capacitors30 and 31 are regarded as a power supply of a converter includingswitching devices, the capacitors 30 and 31, the switching devices 12and 13, and the diodes 16 and 17 are connected in series between the ACpower supply 1 and the load 6. Hereinafter, the series circuit isreferred to as a series converter.

[0100] Also, the capacitors 30 and 31, the switching devices 10 and 11,and the diodes 14 and 15 are connected to the AC power supply 1 inparallel. Hereinafter, the parallel circuit is referred to as a parallelconverter.

[0101]FIG. 2 is a diagram for explaining the principle of the embodimentshown in FIG. 1.

[0102] Referring to FIG. 2, a parallel compensation current source 4represents the parallel converter described above, and a seriescompensation voltage source 2 represents the series converter describedabove. In this case, the series compensation voltage source 2 generatesan arbitrary voltage, so that voltages from two power supplies, i.e. theAC power supply 1 (AC voltage source) and the series compensationvoltage source 2, are added and applied to the load 6. As a result, evenif the voltage of the AC power supply 1 varies and drops, the variablevoltage from the series compensation voltage source 2 is added andapplied to the load 6, so that the voltage drop is compensated and aconstant voltage can be supplied to the load 6.

[0103] Referring to FIG. 3, an operation of the series converter shownin FIG. 1 will be explained further.

[0104] With respect to potentials at points P, M and N shown in FIG. 1,as shown in FIG. 3, a voltage V₃₀ of the capacitor 30 is superposed witha potential at the point M (voltage V_(in) of the AC power supply 1) tobecome a potential at the point P, and a voltage V₃₁ of the capacitor 31is superposed with the potential at the point M to become a potential atthe point N. By controlling the superposed voltages (voltages ofhatching portions in FIG. 3) through chopping of the switching devices12 and 13, it is possible to superpose an arbitrary voltage with thepotential at the point M to maintain a voltage V_(out) at both ends ofthe load 6 at a constant level.

[0105] The chopping through on-off control of the switching devices 12and 13 is conducted by PWM control so that the voltage V_(out) at bothends of the load 6 is maintained at a specified value, or by the PWMcontrol with the voltage added to or subtracted from the potential atthe point M as the specified value.

[0106] Incidentally, in the power supply controlled by PWM, it isobvious that a waveform of the voltage supplied to the load 6 is made asinusoidal waveform in general, and the waveform of the voltage can bemade an arbitrary waveform if necessary.

[0107] In the series of the operations described above, the parallelconverter (parallel compensation current source 4) conducts charging anddischarging operations between the AC power supply 1 and the capacitors30 and 31 to compensate voltage changes (drop and rise) of thecapacitors 30 and 31 caused by the operations of the series converter(series compensation power supply 2).

[0108] As a result, energy supplied to the load 6 flows only through theseries converter, and energy used only for the voltage compensation bythe series converter flows through the parallel converter. Therefore, ascompared with a conventional double converter configuration, it ispossible to reduce loss caused by the parallel converter and improveconversion efficiency of the entire electric power converter.

[0109]FIG. 4 is a block circuit diagram showing a second embodiment ofthe invention corresponding to a third aspect of the invention.

[0110] A main circuit configuration shown in FIG. 4 is substantiallyidentical to that shown in FIG. 1. Features different from FIG. 1 arethat the voltage detecting means 70 is provided for detecting thevoltage V_(in) at both ends of the AC power supply 1, a voltage V_(M)(V_(MN)) at the pint M based on the potential at the point N, a voltageV_(P) (V_(PN)) at the pint P based on the potential of the point N, andthe voltage V_(out); current detecting means 71 is provided fordetecting a current flowing through the reactor 40; and voltage andreactor current control means 72 is provided for controlling the outputvoltage and the current flowing through the reactor 40.

[0111] In the circuit configuration described above, with the voltageV_(in) across the AC power supply 1 detected by the voltage detectingmeans 70 as a reference sinusoidal wave, it is operated such that thecurrent flowing through the reactor 40 and detected by the currentdetecting means 71 follows the sinusoidal wave by the voltage andreactor current control means 72. With this operation, when the circuitshown in FIG. 1 supplies the voltage V_(out) with a sinusoidal waveformto the load 6, a sinusoidal current is made to flow by following thesinusoidal voltage.

[0112]FIG. 5 is a block circuit diagram showing a third embodiment ofthe invention corresponding to a fourth aspect of the invention.

[0113] Features different from FIG. 1 are that a changeover switch 50having first and second changeover contacts 52 and 53 is provided; acommon terminal 51 is connected to one end of the load 6; the seriesconnection point of the capacitors 30 and 31 is connected to the firstchangeover contact 52; and the connection point of the reactor 41 andthe capacitor 33 is connected to the second changeover contact 53.

[0114] In the circuit configuration described above, in a state that thecommon terminal 51 of the changeover switch 50 is connected to a side ofthe second changeover contact 53 (substantially identical to the stateshown in FIG. 1), when an anomaly occurs due to a fault in the electricpower converter while operating the electric power converter shown inFIGS. 1 and 4, the connection of the common terminal 51 is changed fromthe second changeover contact 53 (side of the electric power converter)to the first changeover contact 52 (side of the AC power supply 1), sothat the voltage supply to the load 6 is maintained. It is obvious thatconditions necessary for the changeover of the changeover switch 50 aregiven by a combination of a signal from the control circuit of theconverter and a contact signal.

[0115]FIG. 6 is a block circuit diagram showing a fourth embodiment ofthe invention corresponding to a sixth aspect of the invention.

[0116] In the embodiment, an energy storage element 60 is connectedbetween the points P and N in parallel through charging and dischargingmeans 61.

[0117] While FIG. 6 shows a configuration in which the energy storageelement 60 and the charging and discharging means 61 are added to theconfiguration shown in FIG. 5, they may be added to the configurationsshown in FIGS. 1 and 4.

[0118] A circuit in which the energy storage element 60 and the chargingand discharging means 61 are added to the configurations shown in FIGS.1 and 4 corresponds to a fifth aspect of the invention.

[0119] The charging and discharging means 61 is formed of asemiconductor switch and a magnetic component such as a reactor, and theenergy storage element 60 may include a secondary cell such as a batteryor a flywheel.

[0120] In the circuit configuration described above, when the AC powersupply 1 is working normally, energy is stored in the energy storageelement 60 via the charging and discharging means 61. When sufficientelectric power is not supplied to the load 6 due to an anomaly of the ACpower supply 1, the energy storage element 60 is connected between thepoints P and N via the charging and discharging means 61.

[0121] With this configuration, when the AC power supply 1 is workingnormally, the energy is stored in the energy storage element 60 via thecharging and discharging means 61, and in case of the anomaly of the ACpower supply 1 such as power outage, the energy is discharged from theenergy storage element 60 via the charging and discharging means 61 andsupplied to the capacitors 30 and 31. Accordingly, in case of theanomaly of the AC power supply 1, it is possible to continue to supply aspecified voltage to the load 6 stably by using the parallel converterand the series converter continuously.

[0122]FIG. 7 is a block circuit diagram showing a fifth embodiment ofthe invention corresponding to a sixth aspect of the invention.

[0123] In the fifth embodiment, the charging and discharging means 61shown in FIG. 6 is divided and charging means 62 is connected to bothends of the AC power supply 1; discharging means 63 is connected betweenthe points P and N; and the energy storage element 60 is connected tothe charging means 62 and the discharging means 63 in parallel.Incidentally, the charging means 62, the discharging means 63, and theenergy storage element 60 may be added to the circuit configurationsshown in FIGS. 1 and 4, so that in this case the fifth aspect of theinvention is configured.

[0124] In the circuit configuration described above, when the AC powersupply 1 is working normally, the charging means 62 charges up theenergy storage element 60. In case of the anomaly of the AC power supply1 such as power outage, the energy stored in the energy storage element60 is supplied to the capacitors 30 and 31 using the discharging means63. While the parallel converter and the series converter are operatedusing the energy, the voltage is supplied to the load 6.

[0125] The charging means 62 and the discharging means 63 are formed ofa combination of a semiconductor switch and a magnetic component, andthe one shown in FIG. 6 may be used as the energy storage element 60.

[0126]FIG. 8 is a block circuit diagram showing a sixth embodiment ofthe invention corresponding to a seventh aspect of the invention.

[0127] In the circuit configurations described above, for example, theone shown in FIG. 1, the reactor 41 is changed to a tapped reactor 42.One end of the tapped reactor 42 is connected to the series connectionpoint of the capacitors 30 and 31, and the other end of the tappedreactor 42 is connected to one end of the load 6 not connected to the ACpower supply 1 and the series connection point of the switching devices12 and 13. The one end of the AC power supply 1 not connected to theload 6 is connected to the tap terminal of the tapped reactor 42.

[0128] With the circuit configuration described above, it is possible toreduce the current flowing through the series converter formed of thecapacitors 30 and 31, the switching devices 12 and 13, and the diodes16, 17, thereby reducing switching lose and further improving theconversion efficiency.

[0129] The operation of the parallel converter is the same, therebyomitting the description.

[0130] The circuit configuration using the tapped reactor 42 as shown inFIG. 8 is applicable to the embodiments shown in FIGS. 4 through 7 asdescribed in eight and ninth aspects of the invention.

[0131]FIG. 9 is a block circuit diagram showing a seventh embodiment ofthe invention corresponding to a tenth aspect of the invention.

[0132] In a circuit configuration according to the seventh embodiment,the connection point of the fourth capacitor 33 shown in FIG. 1 ischanged. The fourth capacitor 33 is connected between the connectionpoint of the load 6 and the second reactor 41 and the series connectionpoint of the first and second capacitors 30 and 31.

[0133] The other configurations are the same as those in FIG. 1, therebyomitting the descriptions.

[0134] In the circuit configuration described above, similar to FIG. 1,the capacitors 30 and 31, the switching devices 12 and 13, and thediodes 16 and 17 constitute the series converter connected between theAC power supply 1 and the load 6 in series. The capacitors 30 and 31,the switching devices 10 and 11, and the diodes 14 and 15 constitute theparallel converter connected to the AC power supply 1 in parallel.

[0135] Further, the first reactor 40 and the third capacitor 32constitute an AC filter for suppressing a switching ripple of theparallel converter, and the second reactor 41 and the fourth capacitor33 constitute an AC filter for suppressing a switching ripple of theseries converter.

[0136] An operation of the embodiment has the principle same as that ofthe embodiment shown in FIG. 1. The series compensation voltage source 2(series converter) shown in FIG. 2 generates an arbitrary voltage, sothat the voltages from the two power supplies, i.e. the AC power supply1 (AC voltage source) and the series compensation voltage source 2, areadded and applied to the load 6. As a result, even if the voltage of theAC power supply 1 drops, it is possible to compensate the voltage dropby adding the variable voltage from the series compensation voltagesource 2, so that a constant voltage can be supplied to the load 6.

[0137] The series compensation voltage source 2 can add or subtract thevoltage to or from the AC power supply 1 through the PWM control of theseries converter having the capacitors 30 and 31 as the power supply.When the voltage is added or subtracted, the energy changed due to thecharging and discharging of the capacitors 30 and 31 can be compensatedthrough charging and discharging the capacitors 30 and 31 by the PWMcontrol of the parallel compensation current source 4 (parallelconverter), so that the energy is balanced as a whole.

[0138] As this time, the switching ripple (high-frequency ripple) causedby the PMW operations of the parallel converter and the series converteris removed through the AC filter consisting of the reactor 41 and thecapacitor 33 or by the AC filter consisting of the reactor 40 and thecapacitor 32, thereby preventing leak to the sides of the power supply 1and the load 6.

[0139] Accordingly, in the embodiment, the energy supplied to the load 6flows only through the series converter in the electric power converter,and the energy used only for the voltage compensation by the seriesconverter flows through the parallel converter. Therefore, as comparedwith the conventional double converter, it is possible to reduce theloss of the parallel converter and improve the conversion efficiency.

[0140] Incidentally, in the configuration shown in FIG. 9, the voltagedetecting means 70, the current detecting means 71, and the voltage andreactor current control means 72 shown in FIG. 4 may be added. Theresulting circuit configuration corresponds to an eleventh aspect of theinvention.

[0141] Similarly, in the configuration shown in FIG. 9, the charging anddischarging means 61 and the energy storage element 60 shown in FIG. 6,or the discharging means 63 and the energy storage element 60 shown inFIG. 7 may be added. The resulting circuit configurations correspond toa twelfth aspect of the invention.

[0142] Further, in the configuration shown in FIG. 9, the tapped reactor42 shown in FIG. 8 may be used instead of the reactor 41. At the sametime, similar to FIG. 8, the both ends of the tapped reactor 42 may beconnected to the series connection point of the capacitors 30 and 31 andthe series connection point of the switching devices 12 and 13,respectively, and the tap terminal of the tapped reactor 42 may beconnected to one end of the AC power supply 1. The resulting circuitconfiguration corresponds to a thirteenth aspect of the invention.

[0143] Further, as described above, the tapped reactor 42 shown in FIG.8 may be used instead of the reactor 41 shown in FIG. 9, and, at thesame time, the charging and discharging means 61 and the energy storageelement 60 shown in FIG. 6, or the charging means 62, the dischargingmeans 63 and the energy storage element 60 shown in FIG. 7 may be added.The resulting circuit configurations correspond to a fourteenth aspectof the invention.

[0144] As described above, the present invention is applicable to theelectric power converter of the double converter type in which thearbitrary AC electric power can be supplied to the load from the ACpower supply in the form of the current output through the operations ofthe series converter and the parallel converter. At this time, the otherelectric power converter suppresses the generated lose, therebyimproving the conversion efficiency and reducing the running cost.Further, a part with a lifetime such as an electrolytic capacitor is notnecessary, thereby improving the life of the device and improving thereliability.

1. An electric power converter comprising: a series converter connectedin series between an AC power supply and a load, the series converterhaving a capacitor as a converter power supply thereof; and a parallelconverter connected parallel to the AC power supply; wherein the seriesconverter compensates a change caused in a voltage of the AC powersupply to keep the voltage fed to the load at a certain value; and theparallel converter conducts charging and discharging operations betweenthe AC power supply and the capacitor to compensate the voltage changesof the capacitor caused by the compensating operation of the seriesconverter.
 2. The electric power converter according to claim 1, furthercomprising: a first series switch circuit including a firstsemiconductor switching device, a second semiconductor switching device,a diode connected in opposite parallel to the first semiconductorswitching device, and a diode connected in opposite parallel to thesecond semiconductor switching device, the first and secondsemiconductor switching devices being connected in series to each other;a second series switch circuit including a third semiconductor switchingdevice, a fourth semiconductor switching device, a diode connected inopposite parallel to the third semiconductor switching device, and adiode connected in opposite parallel to the fourth semiconductorswitching device, the third and fourth semiconductor switching devicesbeing connected in series to each other; a series capacitor circuitincluding a first capacitor and a second capacitor connected in seriesto each other; a third capacitor connected in parallel to the AC powersupply; a fourth capacitor connected in parallel to the load; a firstreactor connected between a first end of the AC power supply and aseries connection point in the first series switch circuit, the firstend of the AC power supply being connected to a first end of the load;and a second reactor connected between a second end of the load and aseries connection point in the second series switch circuit; wherein thefirst series switch circuit, the second series switch circuit, and theseries capacitor circuit are connected in parallel to each other tothereby constitute a first parallel connection circuit; the second endof the AC power supply is connected to the series connection point inthe series capacitor circuit; the series capacitor circuit and thesecond series switch circuit constitute the series converter; and theseries capacitor circuit and the first series switch circuit constitutethe parallel converter.
 3. The electric power converter according toclaim 2, further comprising: voltage detecting means for detecting aninput voltage of the electric power converter, an output voltage of theelectric power converter, and a voltage in the first parallel connectioncircuit: current detecting means for detecting a current flowing throughthe first reactor; and controlling means for controlling the outputvoltage of the electric power converter and current flowing through thefirst reactor by using values detected by the voltage detecting meansand the current detecting means.
 4. The electric power converteraccording to claim 2, further comprising: a changeover switch includinga first changeover contact, a second changeover contact, and a commonterminal so that a connection between the second end of the load and thefirst end of the second reactor can be disconnected, wherein the firstchangeover contact is connected to a series connection point in theseries capacitor circuit; the second end of the load is connected to thefirst end of the second reactor via the common terminal and the secondchangeover contact while the electric power converter is workingnormally; and the common terminal is connected to the first changeovercontact when an anomaly is caused in the electric power converter, tothereby feed a voltage from the AC power supply to the load.
 5. Theelectric power converter according to claim 2, further comprising: anenergy storage element; and charging and discharging means connected tothe energy storage element, to thereby feed a voltage to the load usingthe energy stored in the energy storage element when the power supplyvoltage is anomalous.
 6. The electric power converter according to claim4, further comprising: an energy storage element; and charging anddischarging means connected to the energy storage element, to therebyfeed a voltage to the load using the energy stored in the energy storageelement when the power supply voltage is anomalous. 7-9. (cancelled).10. The electric power converter according to claim 1, furthercomprising: a first series switch circuit including a firstsemiconductor switching device, a second semiconductor switching device,a diode connected in opposite parallel to the first semiconductorswitching device, and a diode connected in opposite parallel to thesecond semiconductor switching device, the first and secondsemiconductor switching devices being connected in series to each other;a second series switch circuit including a third semiconductor switchingdevice, a fourth semiconductor switching device, a diode connected inopposite parallel to the third semiconductor switching device, and adiode connected in opposite parallel to the fourth semiconductorswitching device, the third and fourth semiconductor switching devicesbeing connected in series to each other; a series capacitor circuitincluding a first capacitor and a second capacitor connected in seriesto each other; a third capacitor connected in parallel to the AC powersupply; a first reactor connected between a first end of the AC powersupply and a series connection point in the first series switch circuit,a first end of the AC power supply being connected to a first end of theload; a second reactor connected between a second end of the load and aseries connection point in the second series switch circuit; and afourth capacitor connected between the second end of the load and asecond end of the AC power supply; wherein the first series switchcircuit, the second series switch circuit, and the series capacitorcircuit are connected in parallel to each other, to thereby constitute afirst parallel connection circuit; the second end of the AC power supplyis connected to a series connection point of the series capacitorcircuit; the series capacitor circuit and the second series switchcircuit constitute the series converter; and the series capacitorcircuit and the first series switch circuit constitute the parallelconverter.
 11. The electric power converter according to claim 10,further comprising: voltage detecting means for detecting an inputvoltage of the electric power converter, an output voltage of theelectric power converter, and a voltage of the first parallel connectioncircuit: current detecting means for detecting a current flowing throughthe first reactor; and controlling means for controlling an outputvoltage of the electric power converter and a current flowing throughthe first reactor by using a voltage value detected by the voltagedetecting means and a current value detected by the current detectingmeans.
 12. The electric power converter according to claim 10, furthercomprising an energy storage element; and charging and discharging meansconnected to the energy storage element, to thereby feed a voltage tothe load using an energy stored in the energy storage element when thepower supply voltage is anomalous. 13-14. (cancelled).
 15. The electricpower converter according to claim 1, further comprising: a first seriesswitch circuit including a first semiconductor switching device, asecond semiconductor switching device, a diode connected in oppositeparallel to the first semiconductor switching device, and a diodeconnected in opposite parallel to the second semiconductor switchingdevice, the first and second semiconductor switching devices beingconnected in series to each other; a second series switch circuitincluding a third semiconductor switching device, a fourth semiconductorswitching device, a diode connected in opposite parallel to the thirdsemiconductor switching device, and a diode connected in oppositeparallel to the fourth semiconductor switching device, the third andfourth semiconductor switching devices being connected in series to eachother; a series capacitor circuit including a first capacitor and asecond capacitor connected in series to each other; a third capacitorconnected in parallel to the AC power supply; a fourth capacitorconnected in parallel to the load; a first reactor connected between afirst end of the AC power supply and a series connection point in thefirst series switch circuit, the first end of the AC power supply beingconnected to a first end of the load; and a second reactor having a tapterminal and connected between a second end of the load and a seriesconnection point in the series capacitor circuit; wherein a seriesconnection point in the second series switch circuit is connected to thesecond end of the load via a second end of the second reactor; the firstseries switch circuit, the second series switch circuit, and the seriescapacitor circuit are connected in parallel to each other to therebyconstitute a first parallel connection circuit; the second end of the ACpower supply is connected to the tap terminal of the second reactor; theseries capacitor circuit and the second series switch circuit constitutethe series converter; and the series capacitor circuit and the firstseries switch circuit constitute the parallel converter.
 16. Theelectric power converter according to claim 1, further comprising: afirst series switch circuit including a first semiconductor switchingdevice, a second semiconductor switching device, a diode connected inopposite parallel to the first semiconductor switching device, and adiode connected in opposite parallel to the second semiconductorswitching device, the first and second semiconductor switching devicesbeing connected in series to each other; a second series switch circuitincluding a third semiconductor switching device, a fourth semiconductorswitching device, a diode connected in opposite parallel to the thirdsemiconductor switching device, and a diode connected in oppositeparallel to the fourth semiconductor switching device, the third andfourth semiconductor switching devices being connected in series to eachother; a series capacitor circuit including a first capacitor and asecond capacitor connected in series to each other; a third capacitorconnected in parallel to the AC power supply; a first reactor connectedbetween a first end of the AC power supply and a series connection pointin the first series switch circuit, the first end of the AC power supplybeing connected to a first end of the load; a second reactor having atap terminal and connected between a second end of the load and a seriesconnection point in the series capacitor circuit; a fourth capacitorconnected between the second end of the load and a second end of the ACpower supply; wherein a series connection point in the second seriesswitch circuit is connected to the second end of the load via a secondend of the second reactor; the first series switch circuit, the secondseries switch circuit, and the series capacitor circuit are connected inparallel to each other to thereby constitute a first parallel connectioncircuit; the second end of the AC power supply is connected to the tapterminal of the second reactor; the series capacitor circuit and thesecond series switch circuit constitute the series converter; and theseries capacitor circuit and the first series switch circuit constitutethe parallel converter.